To New York City firefighters, who often put in 24-hour shifts, the firehouse can be a home away from home between emergency calls. It is where they cook meals, conduct training, lift weights and trade notes on the latest internal policy or city political imbroglio.

So when the fire commissioner issued an order last month forbidding firefighters to have any "material presenting opinions or viewpoints" in their engine or ladder company quarters, it struck a nerve with the rank-and-file and elicited outrage by officials of the department's largest union, the Uniformed Firefighters Association.

Stephen J. Cassidy, the union president, consulted Ronald L. Kuby, a civil rights lawyer, and put the department on notice that he believed that the order, issued Dec. 29, was a violation of his members' constitutional rights to free expression, even in the workplace.

He then did what he said was a first for his labor organization: He issued a memo on Monday directing roughly 8,000 firefighters simply to ignore the order from the fire commissioner, Salvatore J. Cassano.

"They cannot tell my members that they cannot bring material into the firehouse that has an opinion, because, in effect, not only is that so incredibly chilling, but what is next?" Mr. Cassidy said. "Do they really think they can control what we think? What we read? What we believe?"

On Wednesday, two days after Mr. Cassidy issued his memo, a Fire Department spokesman acknowledged that the order was too broadly worded, and said a new directive had been issued. The spokesman, Francis X. Gribbon, said that fire officials did not intend to keep firefighters from bringing opinion materials into the firehouse; the order was meant to prohibit the posting of opinions on any walls in the firehouses. Posting unofficial materials on bulletin boards has always been prohibited.

But the new language was of even greater concern to Mr. Cassidy, who said he would fight any effort to ban the posting of any written materials that are not official business on the walls of the city's 350 firehouses.

Mr. Kuby agreed, saying that the new directive was still a "considered attempt to make sure that firefighters neither possess nor disseminate nor display material containing viewpoints in the firehouse."

"The new regulations prohibit the display of any material on firehouse walls or in quarters, not just opinions, but things such as the American flag, a photograph of a firefighter who died in service, a notice of memorial," Mr. Kuby said. "They seem obsessed with preventing viewpoints or opinions from being expressed. Millions of New Yorkers entrust their lives to New York City firefighters -- we can trust them to opine responsibly."

Mr. Gribbon said that firefighters could not hang materials on the walls of firehouses since they are workplaces, as well as being city government buildings.

"It is not their home," Mr. Gribbon said. "It is not the U.F.A.'s quarters. This is city property."

Mr. Cassidy said the issue recalled a similar standoff between the union and the department's leadership in 2007, when the department sought to prohibit firefighters from having anything but names and serial numbers on the outside of their lockers. The union fought back, and the department issued new regulations to clarify its position that it meant to stop firefighters from putting obscene, discriminatory or sexually explicit material on lockers.